1. Field of the Invention
The present invention relates to a data communication in an asynchronous broadband code division multiple access (CDMA) system, and more particularly to a method of setting a transport format combination (TFC) of a medium access control (MAC) layer during the data transmission.
2. Description of the Related Art
In brief, an asynchronous CDMA system has three layers. The first layer is a physical layer in charge of a radio transmission. The second layer is composed of a radio link control (RLC) layer in charge of a reliable data transmission, and a medium access control (MAC) layer in charge of a providing simultaneous and effective transmission of a plurality of services. The third layer is composed of a call control (CC) layer in charge of a call determination and release, a mobility management (MM) layer in charge of an authentication and registration of a service user, and a radio resource control (RRC) layer in charge of a radio resource allocation and management.
In the above-described layer structure, the MAC layer converts a logic channel of the RLC layer into a transmission channel and transmits data to a lower layer, or converts the transmission channel into a logic channel and transmits the data to an upper layer. Accordingly, the MAC layer is provided with a MAC-c/sh module in charge of the transmission/reception of a common/shared transport channel and a MAC-d module in charge of the transmission/reception of a dedicated transport channel. The MAC layer is also provided with a transport format combination (TFC) selection module for mapping the logic channel of the RLC layer on the transmission channel of the MAC layer and adjusting the size of the transmitted/received data during the data transmission.
The radio link control (RLC) layer divides/integrates the data received from the upper layer, and transmits the divided/integrated data to the logic channel of the MAC layer. The RLC is classified into a transparent (TR) mode, an unacknowledged (UM) mode, and an acknowledged (AM) mode. The RLC stores the data received from the upper layer in a transmission buffer, and divides/integrates the data stored in the buffer to a size as large as the size of a packet data unit (PDU) and the number of blocks to transmit the divided/integrated data to the MAC layer.
At this time, according to the 3GPP MAC spec (TS 25.321 3.8.0) of the asynchronous CDMA system, the size of the transmissible PDU and the number of blocks for each transmission time interval (TTI) of the RLC layer are set by the MAC layer. The MAC layer receives a transport format combination set (TFCS) and priority information of the respective logic channels from the radio resource control (RRC) layer. Thereafter, if the MAC layer receives the data and buffer occupancy (BO) information to be transmitted from the RLC layer to the MAC layer, it selects a proper TFC from the TFCS based on the BO and the priority of the respective logic channels, and transmits the size of the PDU and the number of blocks that can be used for the next TTI to the RLC layer to set the TFC.
However, the conventional TFC setting process as described above is inefficient in setting the size of the PDU and the number of blocks to be used when the RLC initially transmits the data. In the conventional TFC setting process, in order for the RLC to transmit the data to the MAC at the m-th TTI, the RLC should have already received from the MAC layer at the (m−1)-th TTI. the information on the useable PDU size and the number of blocks. Accordingly, the MAC layer receives the BO information from the RLC at the (m−1)-th TTI. In repeating this process, there is a contradiction in that the MAC cannot inform the RLC the PDU size and the block number of the data to be transmitted at the zeroth TTI.
Setting of a certain initial value in order to solve this problem cannot maximize the performance of the present TFC setting process. Also, the RLC layer cannot send the data by setting the PDU size and the block number by itself. This is because the RLC layer cannot set the transmissible PDU size and the block number since it only knows its own BO information and does not know the BO information of any other RLC layer being simultaneously transmitted.
Thus, according to the conventional TFC setting process, the RLC layer does not transmit the data for the first TTI, but transmits only BO information to the MAC layer during the initial data transmission even in order to set the initial value of the RFC. The MAC layer indicates to the RLC the PDU size and the block number transmissible at the next TTI. However, this process causes a time delay in the transmission of the data, which must be presently transmitted, until the MAC layer transmits the transmissible PDU size and the block number, and thus quality deteriorates during voice and moving picture services.